Machines, including skid steer loaders, track-type tractors, hydraulic tracked excavators, military tanks, and other similar types of heavy construction and mining equipment, are used for a variety of tasks. These machines typically include ratio control devices that transmit torque from an engine to one or more traction devices that move the machine. These ratio control devices generally include a hydrostatic transmission having a common pump and two motors independently driven by pressurized fluid from the pump, or two completely separate pump and motor combinations.
Two independent motors are required for steering of the machine. That is, during a steering event, one or more driven traction devices located on one side of the machine may be caused to rotate at a slower speed than or in a direction opposite to one or more driven traction devices located on an opposing side of the machine to cause the machine to turn. Without independent driving rotations of the traction devices, these types of machine would not be steerable.
Having one pump per motor lowers the cost of the machine and improves machine efficiency and responsiveness. Specifically, when both motors are driven by the same pump, that pump must be sized to supply two times the maximum flow rate of pressurized fluid that any one motor could possibly demand. In this manner, it would be ensured that the machine would respond as the operator desires under all operating conditions. Although this strategy can be effectively implemented to drive and steer such a machine, the large size of the pump required to supply the maximum capacity of two pumps increases the cost of the hydrostatic transmission. And, larger pumps, because of the momentum thereof, typically have lower efficiencies and less responsiveness. Thus, one pump per motor may be a preferred configuration.
Although a preferred configuration, packaging two pumps, two motors, and their associated drive couplings and hydraulic circuitry in the undercarriage of a machine can be difficult and bulky. One transmission design focused on reducing these negative aspects is described in U.S. Patent Publication No. 2005/0166589 (the '589 publication) by Sakikawa published on Aug. 4, 2005. The '589 publication describes an axle driving apparatus for a walk-behind snow blower machine. This axle driving apparatus has a common input shaft for receiving power from a prime mover by way of a belt and pulley arrangement, and two axially aligned drive axles. The common input shaft extends perpendicular to the axles of the snow blower, and drives a pair of stepless transmissions through a gear train. Each of the stepless transmissions are hydrostatic-type transmissions disposed within a common housing that includes mutually fluidly connected pumps and motors. The hydraulic motors are drivingly and independently connected to their respective axles through associated gear trains.
The pumps and motors of the '589 publication are arranged to conserve space. Specifically, the two pumps are symmetrically offset from the common input shaft and oriented in the same direction as the common input shaft. Similarly, the two motors are offset from the two drive axles and aligned in the same direction as the drive axles. This symmetric and perpendicular pump, motor, and shaft arrangement may function to both reduce the size of the axle driving apparatus and common housing in a direction perpendicular to the drive axles, while restraining its expansion in the axial direction.
Although the axle driving apparatus described in the '589 publication may be compact and simple, it may still lack efficiency and applicability. Specifically, because the common inlet connects to the pumps, and the motors connect to their respective drive axles by way of gear trains, some rotational losses may be unnecessarily incurred. These gear trains may also consume valuable space with the common housing. And, because the entire apparatus is connected to the prime mover by way of only a belt and pulley arrangement, the maximum amount of torque transmittable through the apparatus may be less than required to drive a skid steer loader, a track type tractor, a hydraulic excavator, a military vehicle, and other heavy equipment.
The disclosed transmission configuration is directed to overcoming one or more of the problems set forth above.